1. Field of the Invention
The present invention relates to a process for making a circuit board or a lead frame. In particular, the present invention relates to a process for making a circuit board with a conductor pattern formed on an insulating substrate by the subtractive method, or a process for making a lead frame or a fine pattern from a metal plate using a patterning technique.
2. Description of the Related Art
The subtractive method is an inexpensive, simple method and has conventionally been used most widely for fabricating circuit boards. With the recent trend toward a higher integration and a finer structure of semiconductor devices and various electronic appliances, however, this method is disadvantageous when producing a fine conductor pattern for the circuit board.
FIGS. 1(a) to 1(d) are sectional views showing the conventional process of fabricating a circuit board by the subtractive method disclosed in Japanese Unexamined Patent Publication No. (JP-A) 62-115891 or Japanese Unexamined Patent Publication NO. (JP-A) 2-175825, and show the process of forming a conductor pattern, on a resin substrate, by etching. As shown in FIG. 1(a), a board member 3 with a copper foil 2 attached to a resin substrate 1 is prepared. As shown in FIG. 1(b), the copper foil 2 is formed with a dry film resist (DFR) or coated with a liquid resist for masking to thereby form a resist 4. The resist 4 is exposed and developed by a well-known method thereby to form a resist pattern 4b. Next, as shown in FIG. 1(c), an etching solution is applied to etch the portions 4a other than the portions of the copper foil 2 formed with the resist pattern thereby to leave a copper pattern. As shown in FIG. 1(d), the resist pattern 4b is then removed, so that the remaining copper foil portion constitutes a conductor pattern 5.
According to the conventional method of fabricating a circuit board described above, however, as shown in FIG. 1C, each portion of the conductor pattern 5 tends to assume a substantially trapezoidal shape in which the width (a) of the upper part formed with the resist is smaller than the pattern width (b) near to the boundary surface 6 between the resin substrate 1 and the copper foil 2. This is due to the fact that during the progress of the etching process, the etching solution is applied also to the portion immediately under the masking 4 so that the copper foil 2 is side etched. Especially, the boundary surface 6 between the resin substrate 1 and the copper foil 2 generally has a fine unevenness as shown, and therefore it requires considerable time before the etching solution is sufficiently applied to the uneven boundary surface 6. During this time, the etching solution is undesirably applied also to the portion immediately under the masking 4, as described above.
An attempt to reduce the width of each pattern portion 5 or the pitch (c) between adjacent pattern portions would make it difficult to secure a sufficient width especially at the upper part of the pattern 5 far from the resin substrate 1, which in turn makes it difficult to achieve a fine structure.